<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>CSS3动画1</title>
    <style>
        .box{
            width: 100px;
            height: 100px;
            margin:0 auto;
            background-color: yellow;
            transition: all 1s;
            /*当前元素只要有属性发生变化时，可以平滑的进行过度*/
        }

        .box:hover{
            width: 200px;
            height: 200px;
            border-radius: 50%;
            background-color: green;
        }
    </style>
</head>
<body>
    <div class="box"></div>
</body>
</html>